1. Field of the Invention
The present invention relates to an ultrasonic-wave washing unit, ultrasonic-wave washing apparatus, and ultrasonic-wave washing method that are suitably used to wash a to-be-washed object, such as a semiconductor substrate, e.g., a silicon wafer or compound semiconductor wafer, which forms a semiconductor device, or a glass substrate, which forms a liquid crystal display, and a method of manufacturing a semiconductor device and a method of manufacturing a liquid crystal display to which these washing techniques are applied.
2. Description of the Related Art
In a manufacturing process for a semiconductor substrate or glass substrate for liquid crystal display, submicron particles and the like that adhere to the semiconductor substrate or glass substrate must be removed by washing before and after various precision works. Accordingly, an ultrasonic-wave washing method is available in which a to-be-washed object is washed with a detergent that is supplied with high-frequency ultrasonic waves of 500 kHz to 3.0 MHz, which are little damaging.
More specifically, according to this washing method, an ultrasonic-wave diffuser that transmits and diffuses ultrasonic vibration from a vibrator is brought close a to-be-washed surface of the to-be-washed object, and the detergent is fed into the gap between the diffuser and the surface. As this is done, the to-be-washed object is washed with the detergent that is supplied with the ultrasonic waves.
The efficiency of ultrasonic-wave washing can be improved by enhancing ultrasonic-wave energy over the surface of the to-be-washed object. To attain this, a source of ultrasonic-wave oscillation is improved, or ultrasonic waves from the oscillation source are converged to enhance the ultrasonic-wave energy over the to-be-washed surface, as described in Jpn. Pat. Appln. No. 7-283183.
However, the output of the source of ultrasonic-wave oscillation can be increased only limitedly, and a vibrating plate, an ultrasonic-wave vibrator, and an adhesive agent that is used to bond these elements together are limited in life performance and reliability. In order to enhance the ultrasonic-wave energy, therefore, a method is used to converge the ultrasonic-wave output, thereby increasing the apparent value of the output.
Conventionally, as described above, the ultrasonic-wave energy to be applied to the to-be-washed surface is enhanced by converging the ultrasonic-wave output. Recently, however, wires and other patterns formed on substrates have become finer and finer. Therefore, high-frequency ultrasonic waves, which had conventionally been regarded as little damaging, have started to damage the patterns considerably. The inventor hereof closely examined the principle of occurrence of damage, and ascertained that ultrasonic waves generated from the ultrasonic-wave vibrator were converged on a certain point and energy of a level that breaks the wires or influences their crystals was produced on that point. Thus, convergence and synthesis of ultrasonic vibration, which had conventionally been carried out to increase the ultrasonic-wave energy, were found to be the cause of the damage.
If there is a wide gap between the ultrasonic-wave diffuser and the to-be-washed object, the necessary quantity of detergent to fill the gap increases, thus entailing higher cost. Accordingly, the diffuser must be brought as close to the surface of the to-be-washed object as possible. If the gap is too narrow, however, the surface of the object is finely undulating and its thickness is uneven. If the ultrasonic-wave washing unit is moved in a fixed height position, therefore, the diffuser and the to-be-washed object touch each other, so that the wires and the like on the surface of the object may be broken.